In the field of industrial radiography, especially for non-destructive testing applications, any time saving measure is welcome.
After exposure with direct-rontgen rays, industrial non-destructive testing film is automatically processed in a cycle, varying from 8 to 12 minutes, the tendency being to reduce the processing time to a maximum of 5 minutes.
A normal processing cycle is characterised by the following steps: developing at 28.degree. C. and fixing, rinsing and drying at 26.degree. C. The developer is normally composed of three concentrates that should be diluted in the right order: alkaline solution A contains hydroquinone, acidic solution B contains 1-phenyl-3-pyrazolidinone and acidic solution C contains glutaric dialdehyd as hardening agent. The need for the complex three-part packaged developer concentrates has been dictated by the fact that glutaric aldehyd tends to react with 1-phenyl-3-pyrazolidinone, that this pyrazolidinone is unstable in alkaline medium and that glutaric aldehyd tends to polymerize in alkaline medium. The fixer is composed of 2 concentrated solutions, whereby solution A contains the commonly used highly active ammonium thiosulphate as a fixing agent and solution B aluminum sulphate as a hardening agent. Hardening agents are necessary to lead the film through the processor without damages, to reduce the amount of water absorption and, as a consequence, the drying time. This drying time may be considered as the bottle-neck in speeding up the whole processing cycle since the same amount of water has to be evaporated in a shorter time. For rapid processing times shorter than 8 minutes the temperature of the developer would have to be increased in order to obtain satisfactory photographic characteristics and this is to the detriment of the physical characteristics.
In the field of industrial radiography it often happens that in order to examine a welded seam for off-shore applications, e.g. pipe-lines, very rapidly the film is taken out of the processor after an incomplete processing cycle of say 5 minutes and the still wet, incompletely fixed film is examined. As a result failure in the examination is not excluded, as image quality, especially detail rendering, is hardly sufficient because of incomplete processing.
When shortening processing time it is practically impossible to dissolve in the fixing step the non-developed silver halide crystals in a still acceptable short time. This is not only a consequence of the large amounts of silver halides necessary for direct-rontgen applications present in silver halide emulsion layers coated on both sides of the film support but also due to the fact that the silver halides used are silver bromoiodide grains. Bromoiodide grains are used for the following reason. In order to achieve high film speed, which is an indispensible asset especially for direct-rontgen applications, efficient absorption of the exposure radiation is a prime condition. It has been shown emperically that for x-rays the mass absorption coefficient is proportional to a power of the atomic number Z as has been described in the "Encyclopaedic Dictionary of Physics" vol. 7, p. 787, eq. 10, Ed. J. Thewlis, Pergamom Press, Oxford 1957. This strongly disfavours the use of chloride (Z=17) compared to bromide (Z=35) or iodide (Z=54).
The bromide and iodide ions released in the developer inhibit further development of the remaining developable silver halide crystals, so that the regeneration capacity of the developer has to be increased resulting in more consumption of chemicals, a higher cost and more environmental load. From the point of view of ecology the use of a fixer containing ammonium-thiosulphate is disadvantageous. The same applies to the use of hardening agents in the developing solutions and in the fixer as well. The three-part development chemistry and two-part fixing chemistry is also little consumer friendly. In the developer glutaric dialdehyd should be avoided as an undesired ingredient, whereas in the fixer the hardening agent is causing flocculation problems in certain circumstances.